This invention relates to superconductors and more particularly to bulk coatings of niobium germanide (Nb.sub.3 Ge) superconducting compositions having transition temperatures of about 20 K and higher.
There are numerous applications for articles of manufacture comprising an appropriate superconducting coating bonded to a desired metallic substrate. Thus, for example, present concepts for a direct current superconducting power transmission line envisage the use of conductors in a dual capacity to contain the cryogenic coolant and also to carry the superconducting current. In this type of design, a layer of superconducting material surrounds a tube carrying liquid helium or possibly liquid hydrogen if an adequately high superconducting transition temperature can be achieved. The tube must be a good thermal and electrical conductor to provide stability to the superconducting coating. The coating, in turn, must have a high transition temperature (T.sub.c) and critical current (I.sub.c) and maintain good contact with the tube.
The art discloses that the superconducting material with the highest known transition temperature is niobium germanide (Nb.sub.3 Ge) having an A-15 structure. A transition temperature of 22.5 K has been measured using small sections of an extremely thin film (&lt; 0.1 .mu.m) of Nb.sub.3 Ge deposited by sputtering under vacuum conditions. It is also known in the art that bulk coatings of Nb.sub.3 Ge having transition temperatures in excess of 20 K may be deposited on suitable substrates by the coreduction of NbCl.sub. 5 and GeCl.sub. 4 in the presence of hydrogen.
The literature does not disclose niobium germanide superconducting compositions containing oxygen nor does it provide any teaching concerning what the effect of oxygen might be on the superconductivity of bulk coatings of niobium germanide. It has been generally believed, however, that the presence of even small amounts of oxygen in other superconductors having an A-15 structure, as, e.g., the very similar Nb.sub.3 (Al,Ge), is highly detrimental.